William Armstrong of Kinmont or Kinmont Willie was a border reiver and outlaw active in the Anglo-Scottish Border country in the last decades of the 16th century.
Armstrong was captured by the forces of the English Warden of the West March in violation of a truce day in 1596. At the Truce Day all who attended to witness the criminal trials were granted 'safe conduct' for the Day and until the following sunrise. Kinmont, a witness to the trials, was taken against the 'safe conduct' and imprisoned in Carlisle Castle. Walter Scott of Buccleuch ("the Bauld Buccleuch"), keeper of Liddesdale on whose land the arrest had been made, protested to the English Warden, Thomas Scrope, 10th Baron Scrope of Bolton. When Scrope refused to release Armstrong, Buccleuch led a party of men on a daring raid into England and broke Armstrong out of the castle with inside help from the English Grahams and Carletons. Elizabeth I of England was furious that one of her Border fortresses had been broken into at a time when peace existed between England and Scotland. Her relationship with James VI of Scotland was tested to the point where James thought he might lose succession to the English throne. He had been all but promised this and a pension from the English in 1586. Elizabeth demanded that Buccleuch should be handed over to the English for punishment. James was caught between allegiance to the Scots who were adamant Buccleuch had done no wrong in rescuing a man who was captured illegally and his desire to pander to his English benefactor, Elizabeth. Buccleuch was eventually 'warded' in England although no action was taken against him.
Kinmont Willie Armstrong was never recaptured. Legend supposes he died in his bed of old age, sometime between 1608 and 1611.
The story of the raid on Carlisle Castle is told in the ballad "Kinmont Willie" (Child No. 186).